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Phenols in oak leaves

Oak leaves are slow to decompose, but does that mean you should ban them from compost piles and mulch?

You often read that you shouldn’t put oak (Quercus spp.) leaves in the compost, because they’ll be toxic to micro-organisms … or is that rather that they’re too acid? (The proponents of this garden myth never seem to be able to agree on the explanation!) Nor, say these same authorities, should they be used as mulch for the same reason(s). But the whole idea is essentially false or at least, highly exaggerated … but it does contain a pinch of truth, as is often the case with garden myths.

The Truth About Oak Leaves

It’s true that oak leaves contain a lot of tannins, phenolic substances that would be toxic to humans if we ate too much of them … but nobody munches on oak leaves. Tannins in too high a concentration are also toxic to certain herbivores (horses, cows, etc.) … and they’ll avoid eating oak leaves if they have any other possibilities, largely because tannins make the leaves very bitter. There are many micro-organisms that will turn toxic tannins into harmless byproducts … but others that won’t touch leaves until the tannins have been broken down.

So, a bit of truth there, however…

The same tannins are present in most other tree leaves too, not only in those of oaks. Tannins are, in fact, very abundant in forests. It’s tannins, for example, that give the rivers that flow through forested areas their brown tea-like coloration. This happens even in northern forests well beyond where oaks grow. Water rich in tannins remains as drinkable as other similar water sources and harbors a wide range of fish species.

Also, humans regularly consume tannins without any harm. The clear brown color of tea comes from tannins, as does the somewhat astringent taste of red wine. And if distillers age whiskey in oak barrels, it’s so the tannins oak wood gives off enrich their taste.

Fresh oak leaves are acidic, but by the time they decompose, most of that acidity is long gone. Photo: colin grice, Wikimedia Commons

As for acidity, true enough, the freshly fallen oak leaves are certainly acid, but they become less and less so as they decompose. At the end of the process, they end up being, depending on the species, slightly acid to even a bit alkaline! And “slightly acid” is actually the acidity most gardeners want for their garden soil.

As a result, the acidity of oak leaves does no harm to plants when they are used as mulch, nor does it make the soil more acidic than it originally was, much to the disappointment of rhododendron enthusiasts, who often mulch their favorite shrub with oak leaves under the mistaken belief that they will acidify their soil. (Rhododendrons and azaleas are, with blueberries, among the few plants that grow best in very acid soils, but mulching is not going to acidify their soil.)

In fact, if you analyze the soil under large oaks where their own leaves have been allowed to decompose for decades, you’ll find it to be … acidic, neutral or alkaline, depending on the pH (acidity level) of the underlying rock. Even after 100 years of superimposed layers of oak leaves, they will have had almost no influence on the acidity of the soil.

The simple truth is that mulch almost never modifies the pH of the soil underneath, no matter what it is made of.

But Oak Leaves Aren’t Perfect

So, when oak leaves are accused of poisoning soil or compost or being too acid, that’s essentially a myth … but that doesn’t mean oak leaves are necessarily a boon to gardeners.

First, oak leaves are very slow to decompose. Not only do they tend to be tough and leathery compared to most other leaves, but the presence of a lot of tannins does seriously slow down decomposition … and usually what you want if you add leaves to compost is fast decomposition.

Oak leaf mulch.

Also, when oak leaves are entire, they make a poor mulch, as they tend to clump together, forming an almost impenetrable layer that perennials and ground covers have trouble breaking through.

That’s why it’s always best to shred oak leaves before using them. Run them under the lawn mower, vacuum them up with a leaf blower (it will chop up the leaves as it picks them up), pour them into a garbage can and shred them with a string trimmer, or whatever. The method is up to you, but when you do break oak leaves into small fragments, tannins will be largely rinsed out the first time it rains, reducing the so-called toxicity to almost nothing, and bacteria will start to decompose the leaves in earnest.

It’s interesting to note that, in several of the world’s greatest gardens, shredded oak leaves are actually the preferred mulch, as they last a bit longer than other leaf mulches!

So, don’t be afraid to use oak leaves in your compost bin or as an ingredient of your mulch: they are essentially harmless and can even be most useful. Just make sure shred them first!